1. Technical Field
A dual curing silicone polymer has functional groups that cure by two different curing mechanisms. Compositions containing the polymer described herein cure to form products (networks) having comparable structures and strengths regardless of which curing mechanism is employed. The polymer and compositions are useful in electronics applications, for example, as conformal coatings, sealants, and adhesives.
2. Problems to be Solved
Radiation (e.g., ultra-violet, UV) curable coatings rely on line of sight irradiation to effect cure. UV cure is a low temperature, rapid, command cure process, which allows fabricators to produce a large amount of cured materials in a small space with high throughput. This makes UV curing an attractive technique for electronic circuit board manufacturers. However, the complicated architecture of component laden circuit boards makes them less than ideal candidates for UV cure due to the “shadow areas” under the components.
Two chemistries are used extensively in UV curable coatings; one is the acrylates the other is the epoxies. Acrylate based systems are triggered by photo generated radical species. The half-life of the radicals in the photoinitiation and polymerization is relatively short such that the polymerization only occurs when active irradiation is taking place. The epoxy based systems cure via a cationic mechanism whereby a latent acid, generally in the form of an onium salt, is fragmented by UV radiation to give a strong acid, which then initiates the self addition of epoxy groups to form an ether linkage. The reactive cationic centers, have relatively long half-life, such that polymerization can continue for days, depending on the nature of the curing matrices. In rigid systems, the cationic centers become trapped in the matrix, and a post-bake can be used to increase the mobility of the matrix hence the cure. In flexible silicone epoxies the polymerization can continue at room temperature.
Several approaches have been proposed to address the problem of shadow cure. One example is baking coatings. Post baking the circuit board after UV irradiation is a costly second step which defeats the purpose of using UV. Furthermore, heat can initiate the breakdown of onium salts for curing cationic species. Similarly, heat can decompose a radical producing species such as peroxides or hyperperoxides for curing the acrylate based systems.
An alternative approach to addressing the problem of shadow cure is a so-called dual cure material. The conventional route to such materials is via a UV triggered radically initiated acrylate functional siloxane with an anaerobic radical initiator system as a secondary mechanism, or conversely via moisture initiated silicone room temperature vulcanizable (RTV) chemistries such as a methoxy silane functional material as shown below.

The silicone acrylate polymers in this family are typically derived by end capping a linear silanol functional polydimethylsiloxane fluid with a trimethoxyacryl silane. While acrylated silicones are available, they suffer from the drawbacks of 1) being difficult to make via traditional hydrosilylation routes, and 2) the acrylate group is prone to thermal polymerization during synthesis. This route is also limited in the polymer architecture available, there being only a limited number of silanol polymers; typically just linear silanol endblocked polymers. When one of the alkoxy groups on the silane is condensed with the silanol fluid that only leaves two alkoxy groups available for condensation, which reduces the secondary cure speed. In polymers such as that illustrated above, the only design variable is the degree of polymerization (DP), denoted by subscript z. Silicone polymers made via this route also have a tendency to fragment due to a backbiting reaction caused by the condensation catalyst.